This invention relates to a magnetic drive to transmit rotational motion from a motive source into an enclosed space without a direct mechanical connection. More specifically, it relates to blenders, mixers, and like machines, and particularly to devices having a stirrer, impeller, blade, or other tool mounted within a removable cup or container, and rotated by means of a motor located in the stationary base of the machine.
Conventional home blenders and mixers incorporate a mechanically-driven impeller rotatably mounted within a removable blender cup. The base of the cup incorporates a generally circular connection plate with a pattern of projections and/or depressions formed on its lower face that is removably mateable, using a vertical, drop-in movement, with a corresponding pattern formed on a like plate attached to the shaft of a motor housed in a base of the machine. This mechanical coupling between the blender cup and the blender motor requires a rotary seal at the base of the cup between the impeller and connecting plate. This seal is subject to considerable wear and tear over time, as is the mechanical coupling. Because seal failure can result in liquid leaking out of the cup, the seal and bearings in the base of the cup are built to ensure sealing at the expense of friction. The friction produces wear, heat, and loss of power. Moreover, the conventional blender produces much unwanted noise, and the mechanical interlocking coupling between the plates can make it awkward or difficult to remove the cup from, and return the cup to, the base.
Many drink mixers have the drive motor mounted in the base directly under the cup. If overall height is a concern, however, the motor may be positioned off to the side and coupled to the driving shaft by a belt or gear arrangement.
Known home and commercial blenders use conventional a.c. motors. While a.c. motors can be constructed and controlled to provide speed variation, as well as the requisite output torque, a typical such motor is generally bulky, heavy, and not well-suited to electronic speed control, let alone electronic braking.
While d.c. brushless motors are also known per se, they have not been used for blenders or blender/shavers. These motors use a comparatively heavy rotor formed of a sector-like array of permanent magnets. Blending of a mass of shaved or cubed ice and liquid, particularly on start up or during a xe2x80x9cfreeze upxe2x80x9d of a frozen drink, requires a comparatively high torque. D.C. brushless motors are characterized by a low output torque as compared to conventional a.c. motors. They therefore have found use as a motive power source mainly in applications such as fans where a low output torque is acceptable.
A commercially viable blender/shaver for the production of frozen drinks must satisfy a variety of special and important design criteria. It should be compact, both in its footprint and overall height, so as to utilize limited space at a bar efficiently. It ideally has a comparatively low weight. The straight-forward approach of placing a conventional electric motor directly under the blender cup increases the overall height of the machine, and therefore is not typically used. There must also be speed control, typically provided through gearing and electronics, to accommodate different power and speed requirements in different phases of operation. Rapid controlled braking is also important to limit the overall time required to blend, to avoid splashing of the blended material after blending is complete, and for safety. Control of vibration, prevention of overheating, or minimization of wear, ease of maintenance, and durability are also important.
It has also been known that an impeller within a blender cup may be driven magnetically or electromagnetically rather than mechanically. One type of magnetic drive couples a rotating permanent magnet outside a blender cup or the like, to another permanent magnet rotatably mounted in the blender cup. U.S. Pat. No. 2,459,224 to Hendricks; U.S. Pat. No. 2,655,011 to The et al.; and U.S. Pat. No. 5,478,149 to Quigg are exemplary of this approach. Hendricks discloses a magnetically operated stirrer for mixing liquids, in which the stirrer has a magnet mounted at its lower end and within the container for the liquid. Quigg discloses a motor that drives a set of magnets,.via gear box and shaft, to couple to another set of magnets mounted on an agitator.
U.S. Pat. No. 3,140,079 to Baermnann uses a large rotating plate to carry a series of circunferentially spaced magnets that pass under one portion of a much smaller, rotatable conductive disc.
U.S. Pat. No. 1,242,493 to Stringham and U.S. Pat. No. 1,420,773 to Stainbrook disclose electrical drink mixers in which a stator of an a.c. motor surrounds and interacts with a rotor in a blender cup, or in its base. In Stringham, a squirrel cage rotor lies in the plane of the stator windings. In Stainbrook an a.c. rotor is mounted in the base of the blender cup and stator coils are located below the cup. Such split a.c. motor arrangements are limited by the torque, speed control, eddy current loss, and emf interference problems of a.c. motors, as accentuated by the physical separation of the stator windings and the rotor. They do not provide good speed control. They do not utilize a d.c. magnetic field coupling. And the inclusion of the rotor of the motor within the container or cup adds unwanted weight to the cup assembly and makes the cup difficult to handle due to gyroscopic effects if it is picked up while the rotor is still spinning.
If the rotor of a brushless d.c. motor were to be located in the base of a blender cup, the cup would not only become heavy and exhibit a severe gyroscopic effect, but it would also xe2x80x9cstickxe2x80x9d to metal sinks and countertops, and would attract loose metallic implements such as silverware, barware, or coins.
It is therefore a principal object of this invention to provide a drive system that provides reliable, speed-controlled rotary power transmission to a rotatable driven element that is sealed from the source of motive power.
Another aspect is to provide a drive that is automatically clutched to disconnect the drive when the load exceeds a preset value or the driven member is moved from its operating position.
A further object is to provide a magnetic drive offering these advantages, in which the driver element is located in a removable blender cup and the blender cup is easy to insert and remove from the blender and is easy to handle when removed from the blender, e.g., it exhibits no significant gyroscopic effect or magnetic attraction.
Yet another object is to provide a low wear, low maintenance, non-mechanical coupling between motor and drive element, and in particular, one which avoids the high maintenance costs associated with present belt drives and mechanical clutches and brakes.
A still further object is to provide a magnetic drive for a blender or the like with the foregoing advantages which is compact, low in weight, and very easy to use and clean.
Another object is to provide a drive whose operating characteristics can be programmed and which can be braked rapidly and reliably.
In its preferred application as a drive for a blender or other food processing apparatus, the present invention employs an electric motor to rotate a ring magnet, preferably an assembly of two ring magnets with axial poles, that is closely spaced from a disc-shaped drive plate formed of a conductive, magnetizable material. The magnet assembly and drive plate each have matching, circumferentially-arrayed poles. The magnet assembly preferably has a set of an even number of generally pie-shaped, permanent magnet poles or segments of alternating polarity. The drive plate is preferably a thin sheet of a ferrous material such as cold-rolled steel with open-ended radial slots that define the poles and control eddy currents. The magnet assembly produces a sufficiently strong field (flux lines) that despite the spacing, which typically includes high reluctance air gaps, nevertheless induces oppositely polarizing magnetization of the disc poles. This induced magnetization couples the magnet assembly to the plate in order to drive it. In a blender, the drive plate is rotatably mounted in the base of the blender cup and supports a shaft that in turn mounts an impeller. The magnet assembly and the motor are separately housed from the drive plate.
The electric motor is preferably a brushless d.c. motor with stator windings that produce a rotating electromagnetic field that interacts with, and produce a torque on, a rotor that includes a magnet assembly like the one magnetically coupled to the disc. The rotor magnet ring is preferably secured to the drive magnet ring by bonding these magnet rings to opposite faces of a circular cold-rolled steel disc. The rotor, drive magnet ring and drive plate are co-axially aligned when the plate and its associated apparatus, such as a blending cup, are in an operating position. The motor and drive housing preferably has a flat upper wall that extends continuously through the magnet-to-plate gap, as does a flat bottom wall on the blender cup. For a magnet assembly with a field strength on its surface of 1400 gauss, the close spacing for a blender application is preferably about 0.25 inch. Use of a comparatively flat d.c. brushless motor mounted under the driven member gives the motor part of the drive a compact configuration, preferably with a height-to-width ratio of as little as about 1:3.
Viewed broadly as a method, the invention includes the steps of rotating a rotor magnet with multiple circumferentially-spaced poles by interacting the poles with a rotating electromagnetic field. The rotor is in turn coupled to a second drive magnet with a like number of circurnferentially-arrayed poles mechanically coupled to rotate in unison with the rotor. The method further includes the steps of directing the magnetic field of the drive magnet axially away from the rotor to induce oppositely polarized magnetic poles in a conductive drive plate that is rotatably mounted, and closely-spacing the drive magnet from the plate so that the induced poles in the plate follow the poles in the rotating magnet assembly despite the spacing and despite a load that resists rotation. The directing of the magnetic field includes bonding the magnets in a sandwich fashion to opposite faces of a thin steel disc and polarizing the ring magnets axially.
In accordance with another aspect of the present invention, the drive of the present invention can include a gear assembly having one or more gears for transmitting torque from the drive plate to a driven member such as, for example, an output shaft. The gear assembly can include one or more gears sized and arranged to reduce, or to increase, the torque transmitted from the drive plate to the driven member. In a preferred application, the drive and gear assembly are employed to rotate the blade of an ice shaver. The ice shaver can be a stand alone unit or can be incorporated with a blender, such as the blender of the present invention, to form an automatic blender/ice shaver machine for manufacturing frozen drinks.